The present invention relates to combined cycle electric power plants, and more particularly to improved steam turbine valve controls, particularly useful in the operation of a steam turbine having a digital/analog control system.
In the generation of electric power in steam turbine plants including the type described, the turbine inlet steam control valves and the steam turbine bypass valve are controlled automatically by a programmed digital computer having an analog backup control. Certain of the valves, such as the main bypass and the control valves, for example, may be operated sequentially; that is, one of the valves must reach a certain position before the next one opens and closes; or the valves may be operated singly; that is, a number of valves may be operated as one.
In conventional installations, the operating mechanism of the valves is typically controlled by an analog signal, the value of which determines the position of the valve. Also, in systems presently in use an output from the digital computer provides an input to a resistive network which converts the digital signal to an analog signal.
When operating in either the digital or analog mode, it is desirable for the system not in control to track the controlling signal so that the valve positions remain unchanged when transferring from one to the other. This provides a so-called "bumpless" transfer. In such prior systems, a separate tracking scheme is required when operating in the analog mode for transfer to the digital mode. In order to simplify both the analog and digital portions of the control system, it is desirable to have a similar input signal to the interface of the two systems, without the necessity of converting one type of interface input to another.
In the analog portion of the control system, it is desirable to vary the duration of an input to the interface instead of the value of the input, because it can be controlled within very close limits or tolerances. Also, such analog system can use comparative techniques in order to produce the required input to the interface. In the digital portion of the system, an input can occur during a required number of successive cycles to operate the valves to the controlled position. This simplifies both the operation of the two systems and the transfer from one of the systems to another.
Further, it is desirable for the manual control to be able to override the digital control without the necessity of transferring from one mode to another.
In stand alone steam turbine power plants, which are often unattended, a similar kind of digital/analog control can be desirable.